Preforms for resin transfer molding (RTM) are typically composed of layers of oriented fiber material which are assembled in a stack and shaped prior to insertion into a mold for resin injection. Present methods for preparing the layers for molding involve stacking multiple layers of the fiber material with the desired fiber orientation, and then stitching or stapling the sheets together, to maintain the alignment and stabilize the fabric to prevent fraying.
The stack is then cut, outside of the stitching or stapling, to the desired shape usually using a laser cutter. The preform is then placed in a mold and resin injected to impregnate the fabric. These methods for preform stabilization however, are limited to 2-dimensional (flat) structures, and the preform cannot be shaped to conform to a complex part's contour without disturbing the stitching or stapling. If attempts are made to bend the preform, for example, to form a curved surface, the layers may separate or tear at the stitching. These methods are also quite labor intensive, increasing the cost for producing a complex shaped part.
Another method for stabilizing a preform involves spraying a hot melt adhesive onto the layers to hold them together. Usually, the dry fabric is coated in limited areas with a thin layer of the liquified thermoplastic polymer or alternatively, a fibrous polymer is placed between the layers and heat applied to melt the adhesive. The preform is then contoured to conform to the complex shape by melting and refreezing the polymer by the selective application and removal of a hot iron. This, of course, is a highly labor intensive operation.
Since the hot melt adhesive is a thermoplastic polymer, it has several disadvantages. First, the adhesive does not sufficiently wet the fibers to hold the adjacent layers together to maintain a shape after forming. Thus, if the preform is handled, such as during loading into the mold, it is possible that the fiber layers will shift. Also, such a thermoplastic material is not compatible with thermosetting resin systems typically used to produce parts via a resin transfer molding process. During injection, the thermoplastic forms pockets within the part which prevent resin infiltration, reducing part strength. Thus, to form complex composite articles utilizing hot melt adhesives requires sacrificing part strength.